Convert CONFIG_SKIP_LOWLEVEL_INIT et al to Kconfig

This converts the following to Kconfig:
   CONFIG_SKIP_LOWLEVEL_INIT
   CONFIG_SKIP_LOWLEVEL_INIT_ONLY

In order to do this, we need to introduce SPL and TPL variants of these
options so that we can clearly disable these options only in SPL in some
cases, and both instances in other cases.

Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/arch/mips/mach-mtmips/mt7628/lowlevel_init.S b/arch/mips/mach-mtmips/mt7628/lowlevel_init.S
index e4a6c03..83cd8fa 100644
--- a/arch/mips/mach-mtmips/mt7628/lowlevel_init.S
+++ b/arch/mips/mach-mtmips/mt7628/lowlevel_init.S
@@ -28,7 +28,7 @@
 	.set noreorder
 
 LEAF(mips_sram_init)
-#ifndef CONFIG_SKIP_LOWLEVEL_INIT
+#if !CONFIG_IS_ENABLED(SKIP_LOWLEVEL_INIT)
 	/* Setup CPU PLL */
 	li	t0, DELAY_USEC(1000000)
 	li	t1, KSEG1ADDR(SYSCTL_BASE + SYSCTL_ROM_STATUS_REG)
@@ -116,7 +116,7 @@
 	sub	a1, CONFIG_SYS_DCACHE_LINE_SIZE
 	bnez	a1, 3b
 	 nop
-#endif /* CONFIG_SKIP_LOWLEVEL_INIT */
+#endif /* CONFIG_IS_ENABLED(SKIP_LOWLEVEL_INIT) */
 
 	jr	ra
 	 nop